Building Designer Guide
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Roof Trusses BUILDING DESIGNER GUIDE …The Purpose of this Document is to Identify to the Building Designer the Critical Information Required to Complete a Roof Truss Design. Cork Roof Truss Company Table of Contents Table of Contents ................................................................................................................................... 1 Recommended Drawings ....................................................................................................................... 1 First Floor Construction Plan.............................................................................................................. 1 Roof Schematic Drawing .................................................................................................................... 1 Section Drawings ................................................................................................................................ 2 Eaves Detail ........................................................................................................................................ 3 Elevations ........................................................................................................................................... 4 Features & Penetrations ........................................................................................................................ 5 Chimneys ............................................................................................................................................ 5 Stairwells ............................................................................................................................................ 5 Skylights ............................................................................................................................................. 5 Dormer & Attic Space ........................................................................................................................ 6 Water Tanks ....................................................................................................................................... 6 Attic Hatches ...................................................................................................................................... 6 Building Designer Responsibility ............................................................................................................ 7 Building Restraint & Bracing .............................................................................................................. 7 Loadings ............................................................................................................................................. 7 Connection of Timber Roof Structure to Building ............................................................................. 7 Habitable Space in Dormer Roofs ...................................................................................................... 7 Considerations ....................................................................................................................................... 8 Specifications ..................................................................................................................................... 8 Transport Constraints ........................................................................................................................ 8 Roof Trusses | Building Designer Guide Cork Roof Truss Company Recommended Drawings First Floor Construction Plan Plans need to show dimensions to all walls and features that could affect the roof design. Roof plans are used for guidance by the designer. They should show roof pitches, any span direction requirements, and walls that are intended to be built up into the roof space. They should also show other features that impact on the roof, such as attic hatches, chimneys, water tanks, skylights, SVP’s and attic spaces. Roof Schematic Drawing 1 Roof Trusses | Building Designer Guide Cork Roof Truss Company Section Drawings Section drawings are critical for the roof designer to get an understanding of the roof requirements. They should show floor, ceiling & ridge heights. In addition, they should show the supporting wall & eaves levels. Sections also give an indication of the loading on a roof by showing roof coverings & insulation depths. Roof truss centres may also be specified on section drawings. 2 Roof Trusses | Building Designer Guide Cork Roof Truss Company Eaves Detail Eaves information is crucial for the designer to be able to set out the roof correctly. The architectural detail (below left) clearly establishes the critical setting-out dimensions for the truss designer to create the desired eaves detail for the roof trusses (below right). A roof truss design can only be generated by a truss designer once correct eaves details have been provided. Variations to the cavity width and thickness of cladding to the external face need to be taken into consideration when the roof truss setting out is being determined. If there is a specific rafter size required, this should be shown and noted also. Eaves details should contain the following: Wall plate size and positioning Wall plate level and height Cavity wall make-up and any external cladding Roof pitch Soffit width Soffit height Indication of structural ceiling level (underside of truss) Setting out Dimension Either: The vertical dimension from end of overhang to wall plate level or The vertical dimension from top outside corner of wall plate to top of rafter 3 Roof Trusses | Building Designer Guide Cork Roof Truss Company Elevations Elevations are used primarily as a cross check on all the other information supplied. They are used to confirm roof pitches, eaves heights and roof features like chimneys and skylights. On housing sites with multiple standard house types, street elevations are often required to observe how adjacent houses interact. 4 Roof Trusses | Building Designer Guide Cork Roof Truss Company Features & Penetrations Chimneys It’s necessary to specify the size and positioning of chimney shaft(s) in a roof. Stairwells Where a staircase is desired to access the attic space in a roof it’s necessary to specify the: Stairwell positioning Stairwell opening size Ascending direction of staircase Support positions (if required) at the top of the staircase Skylights Where skylight openings are desired it’s necessary to specify the: Size of roof light Position of the opening on plan Position on the slope 5 Roof Trusses | Building Designer Guide Cork Roof Truss Company Dormer & Attic Space Where openings are desired, it’s necessary to specify the: Position of attic walls on plan Position of dormer walls on plan Position of window on plan Dormer shape and pitch Height of attic ceiling Height of dormer ceiling Water Tanks Where water tanks need to be accommodated it is necessary to specify the: Position of the tank Geometry of the tank Maximum weight of the tank Attic Hatches Where access hatches to the attic space are desired it is necessary to specify the: Dimensions of the hatch Positioning of the hatch on plan 6 Roof Trusses | Building Designer Guide Cork Roof Truss Company Building Designer Responsibility Building Restraint & Bracing The roof designer takes responsibility for the bracing of the roof structure. The bracing of the building and therefore the connection of the roof to the structure is also the responsibility of the building designer. Loadings It is the building designer’s responsibility to inform the roof designer of all the loads being applied to the roof structure. The building designer should review the design to ensure the truss designer has accounted for all necessary loadings. In the case of nursing homes or hospital roofs, it is important to specify any required hoist loads on the trusses. Connection of Timber Roof Structure to Building Specifying connections from the roof structure to the building is the responsibility of the building designer. They should ensure that all actions from wind on a building/roof have been considered. Any connection from the roof structure to the building must be reviewed and confirmed by the building designer. Habitable Space in Dormer Roofs If the attic space is intended as habitable living space in the property it is critical at concept stage to ensure the below requirements are fulfilled. This may influence the external wall set-out or the pitch of the roof. 7 Roof Trusses | Building Designer Guide Cork Roof Truss Company Considerations Specifications Inclusion of client specification documents are important to enable the designer to obtain critical information that could have a significant influence of the design and price of a structure, for example: Treatment requirements Timber and metalwork specifications Truss spacing Transport Constraints It is important for a designer to be aware of delivery constraints. Typically, if a large truss must be delivered on a rigid vehicle then the truss is specially modified for transportation purposes. This could have an implication on the cost of trusses and/or the erection procedure. Examples of transportation limits for typical articulated & rigid lorries are illustrated below: Because of these transport height restrictions, ‘top-hat-trusses’ are often required to be used. This means the top section of the truss must be re-fixed on site using ‘field- splice-plates’. 8 Roof Trusses | Building Designer Guide Cork Roof Truss Company 9 Roof Trusses | Building Designer Guide .